1. Field of the Invention
The present invention relates to a pixel circuit which has a current driving type light-emitting device such as an organic electroluminescent device (hereinafter, called “organic EL device”) and a driving device such as a thin film transistor for driving the current driving type light-emitting device. The present invention also relates to a display apparatus having pixels, each pixel being provided with such a pixel circuit, and further to an electronic apparatus having the same. In particular, the present invention concerns a driving circuit, as well as a display apparatus, capable of compensating for deterioration with time of the current driving type light-emitting device and the driving device, as well as to an electronic device incorporating such a driving circuit and a display apparatus.
2. Description of Related Art
As an example of such a display apparatus, a display apparatus of the type using a Thin Film Transistor (hereinafter abbreviated as a TFT) for driving a current driving type light-emitting device such as an organic EL device as a driving device is configured as will be described. That is, a data signal and a scanning signal, each corresponding to an image to be displayed, are respectively supplied to a signal line and a scanning line provided in a display region from a scanning line driving circuit and a signal line driving circuit. On the other hand, a voltage is applied between a pixel electrode and an opposing electrode at each pixel through a driving TFT provided for each of a plurality of matrix pixels in a display region from a common electrode driving circuit and an opposing electrode driving circuit. Then, a current flowing through a current driving type light-emitting device arranged between the pixel electrode and the opposing electrode is controlled by the TFT for driving each pixel in accordance with a data signal voltage supplied from the signal line at the same time as a scanning signal is supplied from the scanning line.
More specifically, for example, a switching TFT is provided for each pixel for supplying a data signal from the signal line to a gate of the driving TFT through a source and a drain when a scanning signal is supplied from the scanning line to the gate. The conductance between the source and the drain of the driving TFT is controlled (changed) according to a voltage (i.e., a gate voltage) of a data signal supplied to the gate. At this time, the gate voltage is retained for a longer time than the period that the data signal is supplied by a retention capacitor connected to the gate. In addition, a driving current is supplied to an organic EL device, etc. through the source and the drain whose conductance is thus controlled, thereby driving the organic EL device in accordance with a driving current.
Especially, the organic EL device equipped with the driving TFT described above is considered promising as a current control type light-emitting device (hereinafter, described as “TFT-OELD”) for realizing a display panel featuring a large size, highly resolution, a wide viewing angle, and low power consumption.
However, for a current driving type light-emitting device such as an organic EL device, a driving current flows through the inside of the device, so that deterioration over time occurs irrespective of scale. For example, with respect to the organic EL device, it has been reported that significant deterioration over time occurs. (Refer to Jpn. J. Appl. Phys., 34, L824 (1995)). The deterioration of the organic EL device over time is broadly classified into two types. One of them is a reduction in current against a voltage applied to the organic EL device. The other is a reduction in a quantity of emitted light against a given voltage applied to the organic EL device or a current flowing therethrough. Additionally, the degree of deterioration over time varies among each organic EL device. Further, for a TFT-OELD, the TFT deteriorates over time because of a current flowing through the TFT as a driving device. For this reason, in a display apparatus employing the TFT-OELD, a problem of deterioration in image quality arises when the organic EL device or the driving TFT deteriorates over time. That is, deterioration in current decrease or a quantity of emitted light decrease leads to degradation of screen luminance, while variation in these decreases cause screen irregularities. Especially, these kinds of deterioration occur depending upon luminescence characteristics of the organic EL device during manufacture, variations in current-voltage characteristics or threshold characteristics of the driving TFT or history of display patterns, and so forth, thus resulting in deterioration in screen quality of an entire display apparatus, and screen irregularities.
In this connection, Japanese Patent Publication No. 05-019234 discloses a conventional art that an EL device is used as a rear light source (backlight) of a liquid crystal display panel to thereby detect the luminance of the EL device in such a manner that the luminance of an entire liquid crystal display panel lightened from the rear by the EL device does not decrease, thereby correcting for deterioration of the rear light source. However, the conventional art relates to an entire liquid crystal display panel, and an EL device is not provided for each pixel as a display device, and is used merely as a rear light source. Therefore, the conventional art substantially differs from the present invention in its applicability. Additionally, the conventional art does not suggest an effective technology for correcting deterioration over time described above in a display apparatus having each pixel equipped with a current driving type light-emitting device such as an organic EL device. Furthermore, the technical problems of increasing the longevity of a display apparatus or improving the display quality by correcting for deterioration over time in a driving TFT or a current driving type light-emitting device in a display apparatus equipped with a current driving light-emitting device at each pixel is not recognized between and by those skilled in the art.